Crystal-chemical features of high-calcium and hypercalcium eudialyte-group minerals (EGMs) from a carbonatite-related rock of the Tamazeght peralkaline complex, High Atlas Mountains, Morocco were studied using electron probe microanalysis, single-crystal X-ray structure analysis, infrared and Raman spectroscopy. The major components of the host rock are calcite, fluorite and EGMs; aegirine-augite is present in subordinate amounts. The specific features of the studied EGMs are chemical heterogeneity, a complex zoning, reaction zones around calcite and apatite inclusions, Na- and Cl-deficiency, high contents of Ca, Mn, REE, Nb, carbonate and H-bearing groups, positive correlation between Nb and Mn and negative correlations between the pairs Fe-Mn and Zr-Mn. These features confirm previous assumptions about the role of infiltration of carbonatite fluid rich in Ca, Mn, REE, Nb, CO2 and H2O and a depletion of Cl in the remaining fluid after the crystallization of sodalite at the expense of nepheline in the formation of carbonatite-related rocks of the Tamazeght complex. The crystal structure of a single-crystal fragment extracted from the Nb-rich zone refined to R-1 = 0.0335 has shown a high degree of ordering of Na, REE and H3O+ and the dominance of Fe3+ at the M2 site with five-fold coordination. The composition of EGMs from the reaction zones around calcite and apatite inclusions corresponds to Mn-dominant (with Mn > Fe at the M2 site) analogue of feklichevite with the simplified formula Ca-3(Na,K)(9)(H3O)(3)Ca6Zr3(Mn2+,Fe3+,Zr)(3)NbSi(Si24O72)(OH,H2O)(5)(F,Cl)(2/3)(CO3)(1/3).

The new mineral delchiaroite, Cu3I(CH3S)(2), has been discovered in La Piana quarry, Colonnata marble basin, Carrara, Apuan Alps, Tuscany, Italy. It occurs as light-yellow acicular crystals, elongated on [010], up to 0.1 mm in length and is brittle, with a light-yellow streak and a greasy luster. In reflected light, delchiaroite is light gray with abundant internal yellow reflections; anisotropism is moderate, with gray tints. Reflectance values for the four COM wavelengths are (lambda (nm) R-min/R-max (%)) 470: 20.6/22.2; 546: 20.9/23.4; 589: 20.7/23.3; and 650: 20.1/22.4. Electron microprobe analysis resulted in the following (average of 12 spot analyses - in wt %): Cu 47.27, I 31.09, S 14.90, C-calc 5.58, H-calc 1.40, and total 100.24. The empirical formula of delchiaroite, calculated on the basis of Cu + I + S = 6 atoms per formula unit, is Cu3.07I1.01(CH3S)(1.92), in accordance with the end-member formula Cu3I(CH3S)(2). Delchiaroite is orthorhombic, space group Pmmn, with a=16.924(10), b=4.099(2), c=5.572(3) & Aring;, V=386.5(4) & Aring;(3), and Z=2. The crystal structure was refined to R-1=0.0962 for 360 unique reflections, with F-o>4 sigma F and 27 refined parameters. It can be described as being formed by electroneutral [Cu3I(CH3S)(2)](0) {100} layers stacked along a. The origin of delchiaroite is related to the supergene alteration of enargite in vugs of Liassic marbles. Chemically, this mineral belongs to the pseudo-binary system CuI-Cu(CH3S) at a 1:2 ratio. It constitutes the first example of natural iodide-methanethiolate, illustrating the interaction between inorganic and organic chemistry. Its name honors the mineral collector Lorenzo Del Chiaro (b. 1951) for his contribution to the knowledge of the mineralogy of the vugs of the Marble Formation belonging to the Alpi Apuane metamorphic complex.

Watanabeite from the Pefka epithermal deposit, northeastern Greece, was examined using single-crystal X-ray diffraction and electron microprobe analysis. The empirical formula of watanabeite is Cu3.93Fe0.10Ag0.01Pb0.23As1.55Sb0.19S4.99. This mineral is orthorhombic, space group Amm2, with unit-cell parameters a = 10.9601(5), b = 14.6498(8), c = 10.3001(5) & Aring;, V = 1653.82(14) & Aring;(3) and Z = 8. The crystal structure was solved and refined to R-1 = 0.0471 for 2108 unique reflections with F-o > 4 sigma(F-o) and 123 refined parameters. The crystal structure of watanabeite can be described as a three-dimensional framework of Cu-centred tetrahedra; cavities of the tetrahedral scaffolding host Cu6S and As-2(Pb,Sb,As)2S7 clusters. On the basis of structural data, the formula of watanabeite could be written as( [III])Cu(3)([IV])Cu(5)As(3)(Pb,Sb,As)S-10 (Z = 4), considering the three independent three-fold Cu sites and the three independent tetrahedrally coordinated Cu sites as aggregated positions. The occurrence of Pb2+ in watanabeite is probably related to the substitution Cu+ + (As,Sb)(3+) = 2Me(2+), where Me = Pb, Fe, Zn and formally divalent Cu. The relationships with tetrahedrite-group minerals are discussed on the basis of the refined structural model, highlighting possible crystal chemical implications of such relationships.

X-ray powder diffraction and multi-wavelength Raman spectroscopy were employed to characterize carbonaceous geomaterials, offering the first nanostructural analysis of rare pyrobitumen (LP) samples from the Lopérec gold deposit in Brittany, France, by comparing them to Karelian shungite (KS) and carbon allotropes samples. The inter-reticular distances d(002) for LP/KS, derived from X-ray diffraction patterns, are 3.57(1)/3.48(1) Å, with crystal thickness Lc(002) and graphitization degree of 1.4/2.0 nm and 6.9/13.1, respectively. Raman band deconvolution indicates graphitic domain sizes of La = 6.7/8.2 nm and graphene-like flake tortuosity Lt = 9.2/11.3 nm. Extensive density functional theory calculations on various 2D nanoflakes accurately predict that the D and G Raman bands may originate from graphene quantum dots, which form part of the nanostructure of these geomaterials. LP exhibits greater structural disorder than KS, along with a lower density (1.60 vs. 1.85 g/cm³), suggesting a lower degree of graphitization, likely due to formation at a lower temperature (∼300 °C). The Lopérec pyrobitumen is believed to result from a redox process involving a CO2-rich, oxidizing hydrothermal solution interacting with a local hydrocarbon source.

Abstract. A new occurrence of pyrite crystals with rhombohedral habit, up to several centimeters in length, is described from the Madan Pb–Zn ore field (Rhodope Massif, south Bulgaria), where it constitutes a late pyrite generation. As observed in the past in other deposits, the ideal rhombohedron is derived from the pyritohedron by suppression of half of its faces (six “polar faces”) around a ternary axis. In studied crystals, together with six main “equatorial faces”, additional minor faces correspond to cube faces as well as polar faces. Such a dissymmetry indicates that the crystallographic point group of these crystals is 3‾, a subgroup of the eigensymmetry 3‾2/m of a rhombohedron taken as geometric face form. Twinning by metric merohedry confirms such a symmetry decrease and permits the definition of this type of pyrite as a dimorph of cubic pyrite, i.e., pseudo-cubic trigonal pyrite (pyrite-R). Twin operations belong to the set of symmetry operations absent in point group 3‾ relative to pyrite symmetry m3‾: reflection about the {100} plane or two-fold rotation about the <100> direction. Four twin types have been distinguished (name, chromatic point group): three contact twins (reflection, m′; rotation, 2′; trapezoidal, (m(2)m(2)2(2))(4)), as well as one penetration twin (crossed, 2′/m′). Composition planes always correspond to {100}, but there are two types of twin interfaces. More complex twinned samples may develop erratically during crystal growth. Other twin variations as well as genetic aspects of such a type of pyrite are discussed.

Two examples of contact twins in pyrite from Peru are described. The first one, from Pasto Bueno ore deposit, shows the pyritohedron {120} as principal form, accompanied by the {111} octahedron and {100} cube as secondary forms, giving a lenticular aspect. (111) is the composition plane, and the twin operation is any one of the three binary axes ⟨110⟩ within this plane. The second one (unknown ore deposit) presents two forms, the octahedron {111} and the pyritohedron {120}; the two crystals in the twin are elongated along [101] and [011], respectively, producing a V profile. It is a reflection twin where the twin plane (110) coincides with the composition plane. These twins are by merohedry. Another contact twin is known in the literature, reported by Gaubert [Bull. Soc. Fr. Minéral. Cristallogr. (1928), 51, 211-212] who described it as a spinel twin, i.e. a reflection twin with twin and composition plane (111); here it is shown that it is actually a rotation twin in which the twin operation is a 180° rotation about any of the three equivalent directions ⟨211⟩, contained in the (111) composition plane. The occurrence of these twins as well as the doubtfulness of the spinel twin in pyrite shows a direct relationship with the structural interpretation based on the pseudo-symmetry of the crystallographic orbits.

AbstractTennantite-(Cu), Cu12As4S13, was approved as a new mineral species from the Layo epithermal deposit, Castilla Province, Arequipa Department, Peru, where it occurs as black metallic anhedral grains, up to 0.1 mm across, replacing enargite and associated with chalcopyrite and vinciennite. In reflected light, tennantite-(Cu) is isotropic, grey with a bluish shade. Reflectance data for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 29.1; 546: 28.4; 589: 27.4; and 650: 25.0. Electron microprobe analysis for holotype material gave (in wt.% – average of 10 spot analyses): Cu 49.32(27), Fe 2.20(12), Zn 0.09(2), Sn 0.03(5), As 19.45(43), Sb 1.94(10), Te 0.02(5), S 27.75(43), total 100.80(20). On the basis of (As + Sb + Te) = 4 atoms per formula unit (apfu), the empirical formula of tennantite-(Cu) is (Cu11.27Fe0.57Zn0.02)Σ11.86(As3.77Sb0.23)Σ4.00S12.57. Tennantite-(Cu) is cubic, I$\overline 4$3m, with unit-cell parameters a = 10.1710(10) Å, V = 1052.2(2) Å3 and Z = 2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1 = 0.0178 on the basis of 263 unique reflections with Fo > 4σ(Fo) and 24 refined parameters. Tennantite-(Cu) is isotypic with other tetrahedrite-group minerals. Previous findings of tennantite-(Cu) are reported and some nomenclature issues, related to the Fe and Cu oxidation states, are discussed. At the Layo epithermal deposit, tennantite-(Cu) is the result of the replacement of enargite under decreasing $f_{{\rm S}_ 2}$ conditions.

Bond valence analysis has been applied to various copper chalcogenides with copper valence excess, i.e. where the formal valence of copper exceeds 1. This approach always reveals a copper bond valence excess relative to the unit value, correlated to an equivalent ligand bond valence deficit. In stoichiometric chalcogenides, this corresponds to one ligand electron in excess per formula unit relative to the valence equilibrium considering only CuI. This ligand electron in excess is 50/50 shared between all or part of the Cu-atom positions, and all or part of the ligand-atom positions. In Cu3Se2, only one of the two Cu positions is involved in this sharing. It would indicate a special type of multicentre bonding (`one-electron co-operative bonding'). Calculated and ideal structural formulae according to this bond valence distribution are presented. At the crystal structure scale, Cu-ligand bonds implying the single electron in excess form one-, two- or three-dimensional subnetworks. Bond valence distribution according to two two-dimensional subnets is detailed in covellite, CuS. This bond valence description is a formal crystal-chemical representation of the metallic conductivity of holes (mixing between Cu 3d bands and ligand p bands), according to published electronic band structures. Bond valence analysis is a useful and very simple prospective approach in the search for new compounds with targeted specific physical properties.

Abstract Although everything seemed clear about the Ag-Sb-S compounds belonging to one of the more deeply studied experimental systems, nature allowed us to discover a new polymorph of Ag3SbS3, which could represent a compound for assessing new technological potentialities. The new mineral species pyradoketosite, Ag3SbS3 (IMA 2019-132), was discovered in the pyrite + baryte + iron oxide ore deposit of the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. It occurs as brittle orange acicular crystals, up to 200 μm in length and 25 μm in thickness, with adamantine luster. Under reflected light, pyradoketosite is slightly bluish-gray, with abundant orange internal reflections. Bireflectance is weak, and anisotropism was not observed, being masked by abundant internal reflections. Minimum and maximum reflectance data for the wavelengths recommended by the Commission on Ore Mineralogy [Rmin/Rmax (%) (λ, nm)] are 32.8/32.9 (470), 30.2/30.7 (546), 29.0/29.6 (589), and 27.5/28.4 (650). Electron microprobe analysis gave (mean of 6 spot analyses, in wt%): Ag 59.81, Sb 22.63, S 17.78, total 100.22. On the basis of (Ag+Sb) = 4 atoms per formula unit, the empirical formula of pyradoketosite is Ag2.996(11)Sb1.004(11)S2.996(15). Pyradoketosite is monoclinic, space group P21/n, with a = 13.7510(15), b = 6.9350(6), c = 19.555(2) Å, β = 94.807(4)°, V = 1858.3(3) Å3, Z = 12. The crystal structure was solved and refined to R1 = 0.063 on the basis of 2682 unique reflections with Fo > 4σ(Fo) and 191 refined parameters. The structure of pyradoketosite can be described as formed by the alternation of {101} layers: an Sb-rich layer, Sb3AgS3, and two distinct Ag8S6 layers. This layered organization allows identifying structural relationships with the wittichenite-skinnerite pair. Pyradoketosite is associated with pyrargyrite, tetrahedrite-(Hg), valentinite, and probable pyrostilpnite in baryte + dolomite + quartz veins embedded in metadolostone. Its name derives from the old Greek words “πυρ” (fire) and “άδόκητος” (unforeseen), because of the unexpected occurrence of this third polymorph of the compound Ag3SbS3.

AbstractTennantite-(Hg), Cu6(Cu4Hg2)As4S13, was approved as a new mineral species (IMA2020-063) from the Lengenbach quarry, Imfeld, Binn Valley, Canton Valais, Switzerland. It was identified as an aggregate of black metallic tetrahedral crystals, less than 0.1 mm in size, intimately associated with sinnerite, and grown on realgar. In reflected light, tennantite-(Hg) is isotropic, grey in colour, with creamy tints. Minimum and maximum reflectance data for COM wavelengths in air are [λ (nm): R (%)]: 470: 29.1; 546: 29.1; 589: 28.5; 650: 27.7. Electron microprobe analysis gave (in wt.% – average of 7 spot analyses): Cu 32.57(42), Ag 6.38(19), Tl 0.29(14), Zn 0.04(5), Hg 17.94(2.27), Pb 0.70(51), As 17.83(61), Sb 0.34(8), S 24.10(41), total 100.19(1.04). The empirical formula of the sample studied, recalculated on the basis of ΣMe = 16 atoms per formula unit, is (Cu4.69Ag1.04Tl0.03)Σ5.76(Cu4.35Hg1.58Pb0.06Zn0.01)Σ6.00(As4.20Sb0.05)Σ4.25S13.26. Tennantite-(Hg) is cubic, I$\overline 4$3m, with a = 10.455(7) Å, V = 1143(2) Å3 and Z = 2. The crystal structure of tennantite-(Hg) has been refined by single-crystal X-ray diffraction data to a final R1 = 0.0897 on the basis of 214 unique reflections with Fo > 4σ(Fo) and 22 refined parameters. Tennantite-(Hg) is isotypic with other members of the tetrahedrite group. Mercury is hosted at the tetrahedrally coordinated M(1) site, in accord with the relatively long M(1)–S(1) distance (2.389 Å), similar to that observed in tetrahedrite-(Hg). Minor Ag is located at the triangularly-coordinated and split M(2) site. Other occurrences of tennantite-(Hg) are briefly reviewed and the Lengenbach finding is described within the framework of previous knowledge about the Hg mineralogy at this locality.

Tsugaruite was originally defined as a lead-arsenic sulfosalt from the Yunosawa mine, Aomori Prefecture, Japan. Until recently its crystal structure remained unsolved and its actual classification in the sulfosalt realm was unknown. Here the refinement of the crystal structure of tsugaruite using single-crystal X-ray diffraction data is reported. The mineral is orthorhombic, space group P2nn, with unit-cell parameters a = 8.0774(10), b = 15.1772(16), c = 38.129(4) Å, V = 4674.3(9) Å3, in agreement with previous studies. The solution of the crystal structure of this mineral revealed Cl occupying a specific position. Chlorine was thus sought and found using the electron microprobe; the average of six spot analyses gave (in wt.%): Pb 68.04, As 12.83, S 18.29, Cl 0.63, total 99.80. The empirical formula, calculated on the basis of Pb + As = 43 atoms per formula unit, is Pb28.26As14.74S49.08Cl1.52. Tsugaruite is an N = 4 plesiotypic derivative of the homologous series of Pb-Sb chloro-sulfosalts having the general formula Pb(2+2N)(Sb,Pb)(2+2N)S(2+2N)(S,Cl)(4+2N)ClN. It has a Cl/(Cl + S) atomic ratio close to that of other known Pb-Sb chloro-sulfosalts (pillaite, pellouxite) and slightly higher than that of dadsonite.

AbstractThe new mineral, zvěstovite-(Zn), ideally Ag6(Ag4Zn2)As4S13, was found in quartz–baryte gangue at the mine dump of the abandoned small deposit of Zvěstov, central Bohemia, Czech Republic. Zvěstovite-(Zn) is associated with tennantite-(Zn), tetrahedrite-(Zn), argentotennantite-(Zn), acanthite and supergene azurite and malachite. The new mineral occurs as rare relic anhedral grains rimmed by acanthite, up to 100 μm in size. Zvěstovite-(Zn) is grey, Mohs hardness is ca. 3½–4, in agreement with other members of the tetrahedrite group; the calculated density is 5.16 g.cm–3. In reflected light, zvěstovite-(Zn) is grey with a greenish tint, without bireflectance, pleochroism or anisotropy. Deep red internal reflections are ubiquitous. Reflectance values of zvěstovite-(Zn) in air (R%) are: 28.5 at 470 nm, 26.9 at 546 nm, 25.5 at 589 nm and 23.8 at 650 nm. The empirical formula for zvěstovite-(Zn), based on electron-microprobe analyses (n = 4), is Ag6.27[(Ag3.90Cu0.38)Σ4.28(Zn1.60Fe0.09Cd0.03)Σ1.72]Σ6.00(As2.26Sb1.48)Σ3.74S12.50. The ideal formula is Ag6(Ag4Zn2)As4S13, which requires (in wt.%) Ag 56.01, Zn 6.79, As 15.56 and S 21.64, total 100.00. Zvěstovite-(Zn) is cubic, I$\bar{4}$3m, with unit-cell parameters: a = 10.850(2) Å, V = 1277.3(8) Å3 and Z = 2. The strongest reflections of the calculated powder X-ray diffraction pattern [d, Å (I) (hkl)] are: 3.1321(100) (222), 2.7125(21) (400), 1.9809(11) (521), 1.9180(31) (440) and 1.6357(15) (622). According to the single-crystal X-ray diffraction data (Robs = 0.051), the crystal structure of zvěstovite-(Zn) agrees with the general features of the members of the tetrahedrite group. Zvěstovite-(Zn) is named after its type locality, Zvěstov; the suffix indicates the dominant divalent C-constituent, according to the approved nomenclature of the tetrahedrite group. It is the As-isotype of rozhdestvenskayaite-(Zn). The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2020-061).

Abstract Several generations of chamosite, including a red variety, occur in the Ordovician hydrothermalized oolitic ironstone from Saint-Aubin-des-Châteaux (Armorican Massif, France). Their chemical re-examination indicates a low Mg content (0.925 < Fe/(Fe + Mg) < 0.954), but a significant variation in IVAl. Minor vanadium is present at up to 1.1 wt.% oxide. Variations in IVAl, the vanadium content and the colour of chamosite are related to the hydrothermal reworking of the ironstone. Taking into account other published data, the ideal composition of chamosite is (Fe5–xAl1+x)(Si3–xAl1+x)O10(OH)8, with 0.2 < x < 0.8 (0.2: equilibrium with quartz; 0.8: SiO2 deficit). The red chamosite (IIb polytype) has a mean composition of (Fe3.87Mg0.23Mn0.01□0.07Al1.74V0.07)(Si2.33Al1.67)O10(OH)8. This chamosite is strongly pleochroic, from pale yellow (E || (001)) to deep orange red (E ⊥ (001)). Visible–near-infrared absorbance spectra show a specific absorption band centred at ~550 nm for E ⊥ (001), due to a proposed new variety of Fe/V intervalence charge-transfer mechanism in the octahedral sheet, possibly Fe2+ – V4+ → Fe3+ – V3+. While the formation of green chamosite varieties is controlled by reducing conditions due to the presence of organic matter as a buffer, that of red chamosite would indicate locally a weak increase of fO2 related to oxidizing hydrothermal solutions.

The plagionite homologous series contains four well-defined members with the general formula Pb1+2NSb8S13+2N: fülöppite (N=1), plagionite (N=2), heteromorphite (N=3), and semseyite (N=4). The crystal structure of several natural and synthetic samples of fülöppite, plagionite, and semseyite have been refined through single-crystal X-ray diffraction, confirming the systematic Pb-versus-Sb crossed substitution observed previously in semseyite and fülöppite. This crossed substitution takes place mainly in two adjacent cation sites in the middle of the constitutive SnS-type layer. The substitution coefficient x appears variable, even for a given species, with the highest values observed in synthetic fülöppite samples. The developed structural formula of the plagionite homologues can be given as Pb2N−1(Pb1−xSbx)2(Sb1−xPbx)2Sb6S13+2N. In the studied samples, x varies between ∼ 0.10 and 0.40. In the ribbons within the SnS-type layer, (Pb∕Sb) mixing can be considered the result of the combination, in a variable ratio, of two cation sequences, i.e. (Sb–Sb–Sb)–Pb–Sb–(…), major in plagionite and semseyite, and (Sb–Sb–Sb)–Sb–Pb–(…), major in fülöppite and, probably, in heteromorphite. The published crystal structure of synthetic “Pb-free fülöppite” is revised according to this approach. It would correspond to a Na derivative, with a proposed structural formula of (Na0.5Sb0.5)(Na0.2Sb0.8)2(Na0.3Sb0.7)2Sb6S15, ideally Na1.5Sb9.5S15. In fülöppite, increasing x induces a flattening of the unit cell along c, with a slight volume decrease. Such a general Pb-versus-Sb crossed substitution would attenuate steric distortions in the middle of the SnS-type layer of the plagionite homologous series. Crystallization kinetics seem the main physical factor that controls such an isochemical substitution.

Biagioniite, ideally Tl2SbS2, is a new mineral from the Hemlo gold deposit, Marathon, Ontario, Canada. It occurs as very rare anhedral crystals up to 65 mu m across associated with aurostibite, stibarsen and native gold in a calcite matrix. Biagioniite is opaque with a metallic lustre and shows a black streak. In reflected light biagioniite is moderately bireflectant and not pleochroic. Under crossed polars it is weakly anisotropic with blueish to light-blue rotation tints. Internal reflections are absent. Reflectance percentages for the four standard wavelengths (R-min and R-max) are 35.9 and 373 (471.1 nm); 34.7 and 362 (548.3 nm); 33.8 and 35.3 (586.6 nm); and 31.5 and 33.7 (652.3 nm), respectively. A mean of four electron microprobe analyses gave: Tl 65.12(31), Ag 3.52(9), Sb 20.22(12), S 10.80(8), total 99.66 wt.%, corresponding, on the basis of a total of 5 atoms, to (Tl1.87Ag0.19)(Sigma 2.06)Sb0.97S1.97-Biagioniite is monoclinic, space group P-c, with a = 11.0895(9), b = 14.3124(11), c = 7.9352(6) angstrom, beta= 96.230(8)degrees, V = 1252.02(17) angstrom(3) and Z = 8. The four strongest powder-diffraction lines [d in angstrom (I/I-0) (hkl)] are: 3.56 (100) (310); 3.37 (75) (231); 3.79 (60) (012); 3.03 (60) (032). In the crystal structure [R-1 = 0.024 for 2655 reflections with I > 2 sigma(I)I, thallium adopts various coordinations extending from quasi-linear to quasi-tetrahedral. Antimony forms Sb-Sb pairs, which lead to the formula [Tl+1](4) [Sb-2](4)(+)[S-2(+)](4). Biagioniite is isostructural with dervillite, Ag2AsS2. The new mineral has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA2019-120) and named for Cristian Biagioni, Associate Professor of Mineralogy at the Department of Earth Sciences of the University of Pisa, Italy.

Abstract The classification of the tetrahedrite group minerals in keeping with the current IMA-accepted nomenclature rules is discussed. Tetrahedrite isotypes are cubic, with space group symmetry I4¯3m.$I\overline{4}3m.$The general structural formula of minerals belonging to this group can be written as M(2)A6M(1)(B4C2)X(3) D4S(1)Y12S(2)Z, where A = Cu+, Ag+, o (vacancy), and (Ag6)4+ clusters; B = Cu+, and Ag+; C = Zn2+, Fe2+, Hg2+, Cd2+, Mn2+, Cu2+, Cu+, and Fe3+; D = Sb3+, As3+, Bi3+, and Te4+; Y = S2– and Se2–; and Z = S2–, Se2–, and o. The occurrence of both Me+ and Me2+ cations at the M(1) site, in a 4:2 atomic ratio, is a case of valency-imposed double site-occupancy. Consequently, different combinations of B and C constituents should be regarded as separate mineral species. The tetrahedrite group is divided into five different series on the basis of the A, B, D, and Y constituents, i.e., the tetrahedrite, tennantite, freibergite, hakite, and giraudite series. The nature of the dominant C constituent (the so-called “charge-compensating constituent”) is made explicit using a hyphenated suffix between parentheses. Rozhdestvenskayaite, arsenofreibergite, and goldfieldite could be the names of three other series. Eleven minerals belonging to the tetrahedrite group are considered as valid species: argentotennantite-(Zn), argentotetrahedrite-(Fe), kenoargentotetrahedrite-(Fe), giraudite-(Zn), goldfieldite, hakite-(Hg), rozhdestvenskayaite-(Zn), tennantite-(Fe), tennantite-(Zn), tetrahedrite-(Fe), and tetrahedrite-(Zn). Furthermore, annivite is formally discredited. Minerals corresponding to different end-member compositions should be approved as new mineral species by the IMA-CNMNC following the submission of regular proposals. The nomenclature and classification system of the tetrahedrite group, approved by the IMA-CNMNC, allows the full description of the chemical variability of the tetrahedrite minerals and it is able to convey important chemical information not only to mineralogists but also to ore geologists and industry professionals.